Add tests for folly/lang/Hint.h

Summary: In preparation of the addition of a new hint.

Reviewed By: Orvid

Differential Revision: D45059034

fbshipit-source-id: 27f640dfbba2577de1e9708d9866395e66dce04f

folly/lang/test/HintTest.cpp

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// We don't bother with "real" checking of any sort of difference in generated
+// code (which would be hard to test, can vary depending on build mode, etc.).
+// It's still valuable to have some syntax checking (in the sense of making sure
+// various calls compile as expected without warning) in one reduced place.
+
+#include <folly/lang/Hint.h>
+
+#include <folly/portability/GTest.h>
+
+using namespace folly;
+
+TEST(Hint, CompilerMayUnsafelyAssume) {
+  compiler_may_unsafely_assume(true);
+  bool falseValue = false;
+  compiler_must_not_predict(falseValue);
+  if (falseValue) {
+    compiler_may_unsafely_assume(false);
+  }
+}
+
+TEST(Hint, CompilerMayUnsafelyAssumeUnreachable) {
+  bool falseValue = false;
+  compiler_must_not_predict(falseValue);
+  if (falseValue) {
+    compiler_may_unsafely_assume_unreachable();
+  }
+}
+
+struct NonTriviallyCopyable {
+  NonTriviallyCopyable() {}
+  NonTriviallyCopyable(const NonTriviallyCopyable&) {}
+
+  NonTriviallyCopyable& operator=(const NonTriviallyCopyable&) { return *this; }
+  int data = 123;
+};
+
+TEST(Hint, CompilerMustNotElide) {
+  int x = 123;
+  compiler_must_not_elide(x);
+  NonTriviallyCopyable ntc;
+  compiler_must_not_elide(ntc);
+}
+
+TEST(Hint, CompilerMustNotPredict) {
+  int x = 123;
+  compiler_must_not_predict(x);
+  NonTriviallyCopyable ntc;
+  compiler_must_not_predict(ntc);
+}